[#3364] Not use VoidChannelPromise in SslHandler to guard against IllegalStateException

Motivation:

SslHandler adds a pending write with an empty buffer and a VoidChannelPromise when a user flush and not pending writes are currently stored. This may produce an IllegalStateException later if the user try to add a ChannelFutureListener to the promise in the next ChannelOutboundHandler.

Modifications:

Replace ctx.voidPromise() with ctx.newPromise()

Result:

No more IllegalStateException possible

handler/src/main/java/io/netty/handler/ssl/SslHandler.java

@@ -457,7 +457,11 @@ public class SslHandler extends ByteToMessageDecoder implements ChannelOutboundH
             return;
         }
         if (pendingUnencryptedWrites.isEmpty()) {
-            pendingUnencryptedWrites.add(Unpooled.EMPTY_BUFFER, ctx.voidPromise());
+            // It's important to NOT use a voidPromise here as the user
+            // may want to add a ChannelFutureListener to the ChannelPromise later.
+            //
+            // See https://github.com/netty/netty/issues/3364
+            pendingUnencryptedWrites.add(Unpooled.EMPTY_BUFFER, ctx.newPromise());
         }
         if (!handshakePromise.isDone()) {
             flushedBeforeHandshake = true;